1. Field of the Invention
This invention relates to a process for producing a zinc-nickel alloy plated steel strip. More particularly, it relates to a process for commercially plating a zinc-nickel alloy on a steel strip at from a low line speed to a high line speed in a consistent manner.
2. Prior Art and Problems
Zinc-nickel (Zn-Ni) alloy plating is several times to ten several times more resistant to corrosion than zinc (Zn) plating in the same coating weight on steel strips. In these years, the zinc-nickel alloy plating is thus used in an increasing amount. In order that a zinc-nickel alloy plating exhibit high corrosion resistance, the plating must be controlled to have a nickel content of 10 to 15% by weight because the best corrosion resistance is accomplished in the range where the alloy assumes the .gamma. phase of Ni.sub.5 Zn.sub.21 solid solution among various Zn-Ni alloy phases. Plating having a composition beyond this range have a too noble galvanic potential and the sacrificial corrosion prevention thereof to the steel strip is rather lowered.
Various plating conditions must be controlled before a steel strip plated with a Zn-Ni plating having a nickel content of 10 to 15% by weight can be consistently prepared in high quality. Generally it is required to keep constant such plating parameters as plating current density, and the composition, flow speed, and temperature of plating solution. Additional control must be done before a large quantity of Zn-Ni plated strip can be commercially produced at a low cost. It is necessary to produce plated strips at a high line speed. It is also necessary to maintain their quality constant despite a variation in line speed and current density.
Plating must be carried out at a high current density in order to deposit a desired Zn-Ni alloy at a high line speed, because the weight of alloy electrodeposited depends on the product of current density and plating time. As the current density becomes higher, the voltage across the anode-to-strip resistance, that is, the resistance of plating solution occupies a larger proportion relative to the entire plating voltage. Then the electric conductivity of the plating solution must be increased in order to reduce the cost of steel strip plating operation.
Several techniques are known in the art which aim at commercial operation of Zn-Ni alloy plating at a high current density. With respect to operating conditions, Japanese Patent Application Kokai No. 55-152194 discloses to set the relative speed of plating solution and steel strip to at least 20 m/min. With respect to plating equipment, Japanese Patent Publication No. 61-21319 discloses a horizontal electrolysis equipment in which the distance between an anode and a steel strip to be plated, that is, anode-to-strip distance is reduced. With respect to plating bath, Japanese Patent Application Kokai No. 61-133394 discloses a plating solution having a certain amount of supporting electrolyte added. With respect to the electric conductivity of plating solution, Japanese Patent Publication No. 61-19719 discloses to add controlled amounts of ZnSO.sub.4 and NiSO.sub.4 to a plating solution to increase the electric conductivity thereof. As to a technique for maintaining a consistent quality independent of a variation in manufacturing parameters such as line speed and current density, Japanese Patent Publication No. 60-106992 proposes to add an ammonium ion to a plating solution to reduce curent density dependency.
These techniques, however, are not fully satisfactory in practice under advanced plating conditions as typified by a high line speed of at least 100 m/min. and a high current density of 100 to 250 A/dm.sup.2 (ampere per square decimeter). The reasons are explained below. First, it is difficult to control the composition of a plating layer under such plating conditions wherein the current density is extremely higher than the normal range of 5 to 10 A/dm.sup.2, because the deposition mechanism changes into an abnormal machanism wherein electrochemically less noble zinc preferentially deposits and thus there deposits a plating layer which has a composition different from that of the plating solution. Secondly, it is difficult to suppress an increase of power consumption due to a rise of plating voltage. In addition, there remain unsolved such problems as the influences of oxygen gas and Joule heat generating during high current density plating on the composition of alloy then plated.
In addition, with a processing line in a manufacturer work wherein current density is changed depending on the width of a steel strip to be plated and the desired weight of alloy deposited, it is more difficult to consistently produce a plating of desired quality because the composition of the plating layer depends on current density.
The influence of oxygen gas evolving in the plating solution during plating means the phenomenon that oxygen gas which evolves in direct proportion to an increase of current density causes the nickel content of a Zn-Ni alloy plating to increase, eventually failing to consistently produce a Zn-Ni alloy plating having a predetermined nickel content. In fact, as seen from FIG. 5 showing the relationship between the amount of oxygen gas contained in plating solution and the percentage increase of nickel content in the plating layer, the nickel content of the plating layer drastically increases when the fraction of oxygen gas contained in plating solution exceeds 10%. For the measurements from which FIG. 5 is plotted, a plating solution which contained 2.9 mol/liter in total of Zn.sup.2+, Ni.sup.2+, H.sup.+ and SO.sub.4.sup.2- ions, 0.2 mol/liter of Na.sup.+, and 0.2 mol/liter of K.sup.+ at pH 1.8 was used.
First, the influence of oxygen gas is described in detail.
The influence of oxygen gas varies with a line speed. It is thus difficult to consistently produce a Zn-Ni alloy plating having a nickel content of 10-15% by weight with line speeds in the range of from 10 to 300 m/min.
An experiment was carried out to examine the influence of oxygen gas. A plating system included ten series connected radial plating cells and an anode of 2 m long. A plating solution contained 4 mol/liter in total of Zn.sup.2+, Ni.sup.2+, H.sup.+ and SO.sub.4.sup.2- ions at pH 2.0. Plating was carried out on a steel strip while passing the solution at a temperature of 60.degree. C. and a flow speed of 0.5 m/sec. The results are plotted in FIG. 3 in which the nickel content was plotted as a function of a line speed for different current densities. As seen from FIG. 3, the plating layer drastically changes its nickel content when the line speed or current density is changed. There were often formed plating layers whose nickel content fell outside the preferred range of from 10 to 15% by weight.
The influence of oxygen gas becomes outstanding particularly with a plating cell which is designed to have a reduced anode-to-strip distance in order to carry out plating at a high current density of 100 to 250 A/dm.sup.2 with a minimal plating voltage as disclosed in Japanese Patent Publication No. 61-21319. Since the plating cell having a reduced anode-to-strip distance generally uses an anode in the form of an insoluble electrode, it is imperative that oxygen gas evolves and an amount of oxygen gas existing on the plating surface increases during plating, as will be described later in further detail.
The influence of Joule heat generating during high current density plating means the phenomenon that the amount of Joule heat generated at a high current density increases the temperature of plating solution so that the temperature of plating solution is not maintained constant between the anode and the strip, failing to consistently produce a plating of a predetermined composition.
An experiment was made to examine the influence of Joule heat during plating. The anode used was 1 m long and spaced a distance of 10 mm from a steel strip. A plating solution having the same composition and pH as that used in FIG. 5 and an electric conductivity of 100 mS/cm was passed at a flow speed of 0.5 m/sec. In FIG. 6, an increase in temperature of the plating solution was plotted as a function of a plating current density. The increase in temperature of the plating solution is a difference between the temperatures of plating solution at the outlet and the inlet of the cell. As seen from FIG. 6, the temperature of plating solution is increased by 4.degree. to 13.degree. C. when the plating current density is 100 to 180 A/dm.sup.2.
As described above, the influences of oxygen gas and Joule heat associated with high current density plating must be overcome in order to commercially carry out Zn-Ni alloy plating at a high line speed in a consistent manner.
It is also necessary to use a plating solution having a low viscosity. The use of a highly viscous plating solution inevitably invites a problem of drag-out that part of plating solution is taken out with a steel strip, resulting in a waste of relatively expensive nickel and hence, an increase of plating cost. The viscous solution promotes the stagnation of oxygen gas in plating solution as mentioned above. For these reasons, a plating solution having a low viscosity is preferred.